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Archive for March, 2014

POTD: Application configuration via Guice binding + Groovy

March 1st, 2014

Often I write my applications with Guice. I also often want to make those applications configurable externally. For example I might inject username and password for that app to talk to another app, I might configure some timeout value, and so on. I make these configuration values available in Guice, so that I can access them wherever I need them. All of this is pretty common in many other places, I’d imagine.

Given that all I’m doing here is to pass configuration values from left to right, I thought it’d be nice if I can write configuration directly as a Guice module by using Guice binder EDSL. Then I won’t have to parse and translate these configuration any more.

And that became my project of the day.

This little library allows you to write Guice binding definitions in a text file:

timeout = 3
bind Payment named "customer" to VisaPayment

From your program, you use GroovyWiringModule to load this configuration file:

Module config = new GroovyWiringModule(new File("/etc/myapp.conf"));
Injector i = Guice.createInjector(
  Modules.override( ... my application's modules ...)
    .with(config))

The end result is that the above script gets translated into the following binding:

bind(int.class).annotatedWith(Names.named("timeout")).toInstance(3)
bind(Payment.class).annotatedWith(Names.named("customer")).to(VisaPayment.class)

Using Groovy as the host language for DSL has other benefits. If you are using system properties or environment variables to configure something, you are basically stuck with strings as the only representation of the configuration. With Groovy, I can create a relatively complex object and bind them, or even put some logic to further obtain values from elsewhere:

bind Payment toInstance new VisaPayment(
  cardNumber: "1234-5678-9012-3456",
  expiration: new Date(System.currentTimeInMillis()+TimeUnit.DAYS.toMillis(30),
  cvv: new URL("http://secret.server/cvv").text) 

With the functionality in Guice to override definitions in one module by another, I can also even override bindings defined in programs, for example to get more logging, add a filter, etc.

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